I wasn't familiar with dictionary.com, but they did a great job of providing a textbook example of how not to define a word. Merriam-Webster has a better definition:
The systematic use of terror especially as a means of coercion.

And in turn, the terror definitions that fit with this are:
1: a state of intense fear
4: violent or destructive acts (as bombing) committed by groups in order to intimidate a population or government into granting their demands

A couple of years ago, there was an astronomy news story about the discovery that our nearest spiral-galaxy neighbor, the Andromeda Galaxy, has two large black holes (with masses of several million sols) in its core. This explained some of the anomalies in that galaxy's shape, which isn't quite as flat or symmetrical as you'd expect a big spiral to be. The suggested explanation is a merger with another galaxy that probably happened several billion years ago, long enough that the resulting mess has settled down into what at first glance looks like a single normal spiral galaxy.

This isn't at all unusual, though. There are lots of galactic collisions visible in space. There was a fun one a few days ago on the Astronomy Picture of the Day [nasa.gov] site. Stories on them generally explain that few if any of the stars collide, because they're too far apart. The dust and gas clouds do collide, and the result is a period of star formation. In many cases, simulations show that the galaxies merge, typically producing an elliptical galaxy if both were large and had different orientations. In the Andromeda case, they were probably roughly coplanar, so the merger just produced a slightly bigger spiral.

Another recent story is about calculations showing that the Andromeda and Milky Way galaxies are on a collision course, and will collide in several billion years. The result may look a lot like the above picture to astronomers in other nearby galaxies.

Astronomers have also found the remnants of several smaller galaxies that our Milky Way has gobbled up. They were generally disrupted, but most of the stars from a single such galaxy now have similar orbits, so each appears as a loose "stream" of stars with a thickening that corresponds to the core of the original small galaxy. It's likely that each such smaller galaxy contributed one or more "medium" black holes (with a few thousand solar masses) to our galaxy.

Anyway, this story isn't especially surprising to anyone who follows atronomy news.